perf stat: Factor out body of event handling loop for system wide

perf stat: Factor out body of event handling loop for system wide

Introduce handle_interval() function that factors out body of event
handling loop for attach and system wide monitoring use cases.

Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/73130f9e-0d0f-7391-da50-41b4bf4bf54d@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>